McKenna's Pharmacology, 2e

33

C H A P T E R 3  Toxic effects of drugs

Hypersensitivity Some people are excessively responsive to either the primary or the secondary effects of a drug. This is known as hypersensitivity and it may result from a pathological or underlying condition. For example, many drugs are excreted through the kidneys; a person who has kidney problems may not be able to excrete the drug and may accumulate the drug in the body, causing toxic effects. The person will exhibit exaggerated adverse effects from a standard dose of the medication because of the accumulation of the drug. In some cases, individuals exhibit increased therapeutic and adverse effects with no definite pathological condition. Each person has slightly different receptors and cellular responses. Frequently older people will react to narcotics with increased stim­ ulation and hyperactivity, not with the sedation that is expected. It is thought that this response is related to a change in receptors with age, leading to an increased sensitivity to a drug’s effects. Hypersensitivity can also occur if a person has an underlying condition that makes the drug’s effects espe­ cially unpleasant or dangerous. For example, a person with an enlarged prostate who takes an anticholinergic drug may develop urinary retention or even bladder paralysis when the drug’s effects block the urinary sphincters. This person needs to be taught to empty the bladder before taking the drug. A reduced dose may also be required to avoid potentially serious effects on the urinary system. DRUG ALLERGY A drug allergy occurs when the body forms antibod­ ies to a particular drug, causing an immune response when the person is re-exposed to the drug. This is con­ sidered an “unpredictable side effect”. A person cannot be allergic to a drug that has never been taken, although people can have cross-allergies to drugs within the same drug class as one formerly taken. Many people state that they have a drug allergy because of the effects of a drug. For example, one woman stated that she was allergic to the diuretic frusemide ( Lasix ). On further questioning, the nurse or midwife discovered that the woman was “allergic” to the drug because it made her urinate frequently—the desired drug effect, but one that the woman thought was a reaction to the drug. Ask additional questions of people who state that they have a drug “allergy” to ascertain the exact nature of the response and whether or not it is a true drug allergy. Many people do not receive needed treatment because the response to the drug is not understood. Drug allergies fall into four main classifications: anaphylactic reactions, cytotoxic reactions, serum sickness and delayed reactions (see Table 3.1). Nurses

and midwives involved in administering drugs must con­ stantly assess for potential drug allergies and must be prepared to intervene appropriately.

KEY POINTS

KEY POINTS

■■ All drugs have effects other than the desired therapeutic effect. ■■ Primary actions of the drug can be extensions of the desired effect. ■■ Secondary actions of a drug are effects that the drug causes in the body that are not related to the therapeutic effect. ■■ Hypersensitivity reactions to a drug are individual reactions that may be caused by increased sensitivity to the drug’s therapeutic or adverse effects. ■■ Drug allergies occur when a person develops antibodies to a drug after exposure to the drug. Drugs can act directly or indirectly to cause many types of adverse effects in various tissues, structures and organs (see Figure 3.1). These drug effects account for many of the cautions that are noted before drug admini­ stration begins. The possibility that these effects can occur also accounts for the contraindications for the use of some drugs in people with a particular history or underlying pathology. The specific contraindications and cautions for the administration of a given drug are noted with each drug type discussed in this book and in the individual monographs found in various drug guides. These effects occur frequently enough that the nurse or midwife should be knowledgeable about the presenta­ tion of the drug-induced damage and about appropriate interventions to be used should they occur. Dermatological reactions Dermatological reactions are adverse reactions involving the skin. These can range from a simple rash to poten­ tially fatal exfoliative dermatitis. Many adverse reactions involve the skin because many drugs can deposit there or cause direct irritation to the tissue. In dark-skinned people, including Indigenous Australians, Ma– ori people and dark-skinned Africans, careful skin assessment is needed as such reactions may not be as evident due to normal pigmentation. For example, redness is not often evident so assessment will be based upon skin tempera­ ture, localised swelling or tightness. Rashes, hives Many drugs are known to cause skin reactions. Older drugs such as procainamide, used in the past to treat DRUG-INDUCED TISSUE AND ORGAN DAMAGE